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LINE POWER SUPPLY CIRCl I 103
terminals must be reversed, or such other changes made as may be required to ground the positive side of K'a wiring to K's case after K has been disconnected from D-C and before it is reconnected to (
(11) Similarly, neither connection to the 220volt motor, M, may be grounded. If either is. cither Genera tor A or Generator B will be short-circuited through the ground. If M needs grounding, its casing may be grounded, but only provided the same be thoroughly insulated from its wiring.
Three-Phase A.C. Supply
(12) Three-phase a.c. 110-220 volt circuits must not be confused with three-wire Edison circuits. They resemble each other only that each has three wires and affords both 1 10 and 220volt service.
(13) Fig. 45 A represents three a.c. generators, 1, 2 and 3, each producing a 60-cycle current at 110 volts. Their three outputs are each 5/3 rd cycle out of phase with reference to the other. The external connections from all three generators, wired as shown, thus constitute a three-phase system. More commonly such systems draw current from a single generator equipped with three sets of coils to give the same type output as indicated in Fig. 45 A. Internal -wiring of such three-phase generators is skeletonized at Figs. 45 B and 45 C.
(14) Note particularly that although Fig. 45 shows a 110-220 volt system, there is no pair of wires from which 220 volts can be drawn. Connection to wires A and C, Fig. 45 A, gives only 110 volts, since these wires are connected directly to the 110-volt output of Generator 2. Connection to any pair of wires, A-B, A-C or B-C, gives 110 volts only. Each pair is directly in parallel with the output of one 110-volt generator. However, when all three wires are connected to a motor or a transformer having internal wiring arrangements as at Fig. 45 B or 45 C, the effective overall pressure applied to that motor or transformer will not be 330 volts (as might be thought) but 220 volts.
(15) The arrangement of the generators in Fig. 45 A,